The present invention relates to a monolithic integrated circuit for an RC oscillator as is known in principle from German Offenlegungsschrift No. 1,921,035.
In the known RC oscillator, the capacitor is charged and discharged between the two voltage values corresponding to the maximum and the minimum oscillator voltage. One terminal of the capacitor is grounded, while the other is connected to a terminal of the integrated circuit and, via a current source, to the live terminal of the supply-voltage source. The known RC oscillator includes a switching stage with a threshold corresponding to the maximum oscillator voltage. Connected in parallel with the capacitor is a discharge-current source which may comprise at least one transistor. In the simplest case, the collector-emitter junction of a switching transistor is connected across the capacitor, which switching transistor is turned on when the voltage across the capacitor reaches the threshold of the switching stage.
In the known RC oscillator, however, the aforementioned switching stage with a threshold corresponding to the maximum oscillator voltage is designed as a threshold circuit with two switching thresholds. The switching stage connects the discharge circuit when the capacitor voltage reaches the upper threshold of response, and disconnects the discharge circuit when the capacitor voltage reaches the lower threshold of response. In an embodiment of the known RC oscillator, the threshold circuit comprises a voltage divider determining the voltage of the upper threshold of response, with one divider resistor being shunted, at the instant the threshold circuit responds, by an additional resistor which is disconnected again when the capacitor voltage reaches the lower switching threshold so obtained.
Such threshold circuits, whose thresholds are preset by means of a voltage divider, usually consist of a differential amplifier having the voltage-divider tap connected to one of its inputs. The lower limit of the lower threshold of response is determined by the base-to-emitter threshold voltage of the differential-amplifier transistor connected to the tap. Especially in the case of circuits having only one supply-voltage source, however, the minimum threshold of response is higher because of the emitter resistor common to both transistors of the differential amplifier.
With the known circuit, it is therefore impossible to utilize the full supply-voltage range for the oscillator voltage because the minimum oscillator voltage cannot be lowered close enough to ground potential.
The smaller the supply voltage available, the more significant this disadvantage becomes. For example, in battery-operated equipment, the ratio of the attainable swing of the oscillator voltage to the battery voltage deteriorates noticeably towards smaller battery voltages.